How the Combustion Engine Works (and Why It Matters)

How the combustion engine works: by detonating hundreds of small explosions every second.

A piston glides along a cylinder, pushing and pulling. The piston pulls down air and gas in the intake stroke and fills the cylinder with the fuel for the fire. The cylinder is primed and the air-gas mixture is ready for compression. Compression creates massive potential energy.

The piston glides back up the cylinder, pushing the air-gas mixture hard and fast into a tiny space—compressing it down to 1/10 (or greater) of its original size (a compression ratio of 10:1). The gases are compressed during the compression stroke, smashing the molecules closer together and increasing the potential energy. Then comes the crescendo.

Spark. Ignite. Explosion. When the gases have been compressed and the piston is at top dead center (TDC), a spark flashes and sets the highly energized compressed gases on fire. Boom! The power stroke—the tiny explosion that propels all combustion engines.

The explosion pushes the piston back down and the crankshaft spins in unison. The energy moves the crankshaft and creates the power under the pedal. Finally, the piston glides back up the cylinder to eject the exhaust—the exhaust stroke—and clears the way for the process to start over. And over. Hundreds of times per second.

Your car’s engine sounds pretty smooth, but slow it down and it sounds like artillery fire—individual booms. An 8 cylinder car idling at 750 RPMs “fires” 50 times per second. That’s 50 power strokes each second while idling!

The basic process of the four-stroke engine is the same as it was in 1876 when Nikolaus Otto invented it. The only differences between an engine that powers the 2017 CTS-V with 640 horsepower (HP) and the 3 HP engine created in 1876: efficiency and capacity. It boils down to compressing air and gas into a tiny space, lighting it on fire and transferring that energy into mechanical motion. That’s all it takes to move the family minivan or James Bond’s Aston Martin.

The Four Basic Strokes: A Closer Look

The entire process only takes four strokes of the piston.

  • Intake Stroke
  • Compression Stroke
  • Power Stroke (Combustion Stroke)
  • Exhaust Stroke

Intake StrokeAs the piston moves down on its first stroke, the intake valve opens and allows the combustion chamber to fill with air and fuel. This creates a space ready to explode even without compression. But add compression and you create a lot of potential energy. It’s possible to add more fuel and more air during the intake stroke to create a more powerful mixture to increase horsepower but be sure to check your tuning if you do add performance parts.

Compression Stroke

When the intake stroke ends at the bottom of the cylinder, the compression stroke begins. The piston moves up and forces the air and gas into a tiny space. A 10:1 compression ratio will compress the air-gas mixture down to 1/10 the size of the compression chamber. The higher the compression ratio the more potential energy which means more power for your pedal. Compress for power. Note that higher compression ratios require higher octane fuels.

Power Stroke (Combustion Stroke)

The power stroke starts when the compression stroke hits top dead center (TDC) (or before top dead center, BTDC) and a perfectly timed spark ignites the gases. This moment is what the engine is all about. Getting the right gases (air-fuel ratio) ignited at the right time with as much energy as possible. The size of the chamber, the compression ratio, the spark, the timing—everything is to maximize the energy of the explosion and then transfer it to mechanical motion. The power stroke thrusts the piston downward propelled by the explosion leading to the fourth stroke.

Exhaust Stroke

On the piston’s second upward stroke, the exhaust valve opens and the piston pushes the burnt gases up and out of the exhaust valves. When your exhaust system is maximized, the pressure the piston encounters on its way up is minimal and the valve moves with less restriction which can give you power gains. Also, with a high performing exhaust system, the exhaust stroke is more likely to push all the burnt gases out of the chamber clearing the way for a clean intake stroke and a stronger power stroke. The cycle is complete and the piston and chamber are ready to begin again.

So What?

If you know how your engine produces power, harnesses that power, works more efficiently at keeping that power, then you’ll know how to make your engine perform its best. You can make cost-effective decisions on which modifications will give you the most horsepower return. To help with engine performance, you might consider an engine tune, a cold air intake, a performance exhaust, a performance intake manifold or even a supercharger.

Now that you know more about the four-stroke process, you will be able to determine how each modification affects each stroke. One mod may affect more than one stroke and give you more bang for your buck.

Leave a Reply